1. Field of the Invention
The present invention relates generally to methods of fabricating three dimensional structures and, particularly, to methods of making three dimensional structures using a lithographic, layer-by-layer process. Such structures include porous three dimensional structures for use in applications where a reduced foreign body capsule formation and increased adjacent vascularization is desired, such as medical devices for permanent and temporary implantation.
2. Description of the Prior Art
Implantable medical devices with biological components are used for various purposes, such as indwelling chemical sensors, controlled drug-release systems, and biohybrid artificial organs for use with cellular therapies. See, for example, Colton, Implantable Biohybrid Artificial Organs, 4 Cell Transplant 415-36 (1995). All of these devices have in common the need for adequate perfusion of small and large molecules to or from the blood stream through the surrounding soft tissue. A serious problem in the development of devices for these applications is the formation of an avascular fibrous capsule around the implanted device. The capsule consists of (i) a layer of macrophages and/or foreign body giant cells at the material-tissue interface, overlain by (ii) an avascular region up to 100 μm thick containing layers of fibroblasts embedded in a collagen matrix, which in turn is overlain by (iii) a region of blood vessels and fibroblasts in a loose connective tissue matrix. Spector, et al., The Local Tissue Response to Biomaterials, 5 Crit. Rev. Biocompat. 269-95 (1989). This capsule creates extra diffusion distance between the vasculature and the device. In addition, the tissue capsule may have inherently poor transport properties, as evidenced by measurements of glucose permeation through fibrotic tissue capsules formed on silicone rubber implanted subcutaneously in rats. The effective diffusion coefficient though this capsule is estimated to be one to two orders of magnitude lower than the value in water, Freeman, et al., A Study of the Mass Transport Resistance of Glucose Across Rat Capsular Membranes, 110 Mater. Res. Soc. Symp. Proc. 773-78 (1989). This reduced diffusion of nutrients and oxygen through the foreign body fibrous capsule has deleterious effects on the viability and/or function of tissues implanted in a biohybrid artificial organ.
Brauker discovered that certain microporous materials, when implanted subcutaneously, induce permanent neovascularization at the interface with host tissue by virtue of their morphology and microarchitecture. Brauker, et al., Neovascularization of Synthetic Membranes Directed by Membrane Microarchitecture, 29 J. Biomed. Mat. Res. 1517-24 (1995). This result was observed with membranes made from a variety of polymers using diverse fabrication methods, including solvent evaporation and stretching. The fact that this behavior was observed for membranes of widely varying chemical composition indicates that microarchitecture, rather than chemistry, is of primary importance in stimulating macrophage migration and neovascularization. Light microscopy revealed that the materials that induce neovascularization have interstices or openings that allow host inflammatory cells, such as monocytes and macrophages, to invade the membrane. Furthermore, once inside the membrane, many of these cells retain a non-flattened morphology and do not adhere to the very thin structural elements of the material. A fibrous capsule overlying the vasculature at the interface may also form around these materials. Brauker observed that materials that produce a thick fibrous capsule without neovascularization at the material-tissue interface had either interstices which were too small for host inflammatory cells to invade, or interstices which were large enough for virtually all of the host cells that invade the membrane to adhere and flatten on the internal structural elements of the material, which provided sufficiently large internal area for cell adhesion. Brauker generally found an increase in inflammatory cell penetration and an increase in vascular structures adjacent to the membrane when the nominal membrane pore size was about 1.0 μm or larger.
Further, Padera demonstrated that the major events in the process of membrane microarchitecture-driven neovascularization occur within the first week of implantation. Padera, et al., Time Course of Membrane Microarchitecture-driven Neovascularization, 17 Biomaterials 277-84 (1996). Host inflammatory cells migrate into the membrane after three days of implantation. Their number increases for seven days, remains constant through 21 days and decreases by roughly half at 329 days. Blood vessels are found closer to the material-tissue interface with increasing time over the first week post-implantation. The vessels first arrive at the interface after three days, increasing rapidly through ten days, and then increase slowly through 21 days. The density of close vascular structures at the interface remained virtually constant after 21 days through 11 months, the duration of Padera's experiment. Fibrous capsule formation starts as early as seven days post-implantation, and the capsule continues to mature until the fibroblasts die or migrate away to leave a nearly acellular, scar-like collagen matrix.
These results correlate with the course of events seen in normal wound healing. In normal wound healing, neutrophils are the predominant cell type at the site of injury within the first 24-48 hours, killing and phagocytosing any bacteria present. The macrophage becomes the predominant cell after this time, removing cellular and foreign debris from the area. Within three to four days, fibroblasts migrate out of the surrounding connective tissue into the wound area and begin to synthesize collagen, which quickly fills the wound space. New blood vessels begin to grow into the area at this time to supply oxygen and nutrients needed by the metabolically active fibroblasts and macrophages in the wound. An important difference between normal wound healing and membrane microarchitecture-driven neovascularization is that in normal wound healing the vessels begin to regress in the second week, but in membrane microarchitecture-driven neovascularization the vessels remain at the interface. Although the mature scar is avascular and acellular in a normal wound, in membrane microarchitecture-driven neovascularization, a multitude of vessels persist at the material-tissue interface in an otherwise largely acellular scar. This persistent adjacent vascular structure would be useful for maintaining the nutrient and oxygen supply to, and thus the viability of, the biological components of artificial organ devices.
These initial experiments which demonstrated the neovascularizing microarchitectural effect used membranes whose surface structure size and spacing were randomly generated, thereby producing an irregular structure. Commonly owned U.S. Pat. No. 5,807,406 (the disclosure of which is incorporated herein by reference) describes a microfabricated porous laminar structure for holding living cells composed of net-like layers of polymer with precisely defined and periodic holes. Although these structures are regular within the two dimensional plane of their laminar layers, they are irregular and sometimes compressed in the third dimensional plane. This creates a less well defined structure in which some interstices are blocked by strands of the polymer net from adjacent layers. Although these structures were also found to generally promote neovascularization at the structure/tissue interface upon implantation into animals, the “blocked” interstices did not allow invasion of those portions of the structure by inflammatory cells.
U.S. Pat. No. 5,797,898 discloses implantable microchip drug delivery devices for controlling the rate and time of release of multiple chemical substances and molecules. Other systems and methods in the prior art disclose biocompatible structures for implantation in general, but fail to disclose structures that can be precisely formed in multiple dimensions. For example, some prior art techniques rely on biocompatible foams for fabricating an implantable structure. For some applications, such structures are sufficient. In other applications, however, precise control of the various internal structural dimensions is important, and such foams provide insufficient dimensional control.
Commonly assigned U.S. patent application Ser. No. 09/731,486, filed Dec. 7, 2000, by Pekkarinen and Brauker (hereinafter “the Pekkarinen et al. application”), discloses a porous, three dimensional structure for use in applications where a reduced body capsule formation and increased adjacent vascularization is desired. The Pekkarinen et al. application reflects substantial improvements over the above-described art. The entire disclosure of the Pekkarinen et al. application is incorporated herein by reference.
It is also known in the art to use rapid prototyping and stereolithographic techniques to create three dimensional structures. One prior art process involves creating a mask layer for a broad field exposure of resin for each layer and requires a large, complex machine. Other approaches use so-called “laser-writing” of resins to create each layer, with the layers created lowered to accommodate the next resin layer. These prior art techniques, however, do not allow for layer-by-layer fabrication having sufficient control over layer thickness and layer feature resolution (e.g., with respect to the two-dimensional pattern reflected in that layer) to produce structures similar to those disclosed in the Pekkarinen et al. application.